Recover Health by using recovery items. For details, please see "How to restore hunter's Health?" below. 

How to restore Hunter’s Health?

• Health can be checked through the Health bar displayed above the hunter's head on the 3D Map View and in the center-bottom of the Hunting Screen. (Note: The Health bar is not displayed above the head when health is full in the 3D Map View.)
• If your Health is below a certain level, you won't be able to start a hunt. Make sure to recover your health before engaging in a hunt.
• Your health will automatically replenish over time in the Main Screen or can be restored by using a recovery item.
• There are 2 types of recovery items: the First-Aid Med and Potion.
• First-Aid Meds will be automatically replenished until you reach Hunter Rank 11, so use it frequently. Even after you become a full-fledged hunter, First-Aid Meds will be supplied in a certain quantity every day, but they will not be replenished beyond the storage limit for First-Aid Meds. 
• Potions can be obtained as rewards and can also be purchased in the shop (HR11 and above). There is no storage limit for Potions.
• Both First-Aid Meds and Potions can be used when hunting monsters, as well as on Main Screen and Hunt Prep Screen.
• During a hunt, First-Aid Meds will be consumed before Potions. Once you use a recovery item during a hunt, you won't be able to use another one for a while.